Electron guns are used to generate a directed stream of electrons with a predetermined kinetic energy. Electron guns are most commonly used to generate electron beams for vacuum tube applications such as cathode ray tubes (CRTs) found in televisions, game monitors, computer monitors and other types of displays.
Many medical and scientific applications require the generation of electron beams as well. Electron guns provide the electron source for the generation of X-rays for both medical and scientific research applications, provide the electron beam for imaging in scanning electron microscopes, and are used for microwave generation, e.g., in klystrons. Commonly, the electron gun is incorporated into a linear accelerator system, or LINAC. LINACs have many industrial applications, including radiation therapy, medical and food product sterilization by irradiation, polymer cross linking and nondestructive testing (NDT) and inspection.
In addition, an electron gun is a key component of the injector system of any high-energy particle accelerator system. The creation of high average-current, high brightness electron beams is a key enabling technology for these accelerator-based systems, which include high-energy LINACs such as Energy-Recovery LINAC (ERL) light sources, electron cooling of hadron accelerators, high-energy ion colliders, and high-power free-electron lasers (FELs). For these applications, the electron gun generates and provides a charged particle beam for input to the accelerator. The output of the accelerator system is an accelerated beam at the energy required for the particular application.
For a growing number of high-power accelerator-based systems, the development of a high average-current high-brightness electron beam has become a major challenge. The electron gun of the injector system must also be capable of delivering short-duration pulses of electrons, i.e. short bunch lengths, at a high repetition rate, preferably in a continuous-wave (CW) mode. These requirements have not been realized by conventional electron gun designs, which suffer from unacceptable degradation in efficiency, reliability and lifetime.
An electron gun, also referred to as an injector, is composed of at least two basic elements: an emission source and an accelerating region. The emission source includes a cathode, from which the electrons generated in the emission source escape. The accelerating region accelerates the electrons in the presence of an electric field to an accelerating electrode (anode), typically having an annular shape, through which the electrons pass with a specific kinetic energy. Typical injectors deliver all of the electrical current from a single cathode, which is incorporated into the accelerating region. The commonly known cathodes used in electron guns generate electrons either by thermionic emission, field emission, or photoemission.
Thermionic emission cathodes emit thermally-generated electrons. These cathodes are typically used in applications with low power requirements, for example, as the electron beam source in electron microscopes. Capable of reaching current densities of only about 20 Amps/cm2 and unable to provide short pulses, these cathodes are inappropriate for use in high-current electron guns for high-power accelerator-based systems. In addition, thermionic emitters are easily contaminated.
The field emission cathodes currently known are likewise inadequate, because they can not deliver high-brightness, or equivalently, low-emittance electron beams in an efficient manner. The high field strengths (at least 1 MV/m) required to obtain reasonable emission make these cathodes impractical for reliable and efficient use in accelerator applications.
Photoemission cathodes have been used in electron guns, commonly referred to as photoinjectors, with some success for accelerator-based systems. Photoinjectors are known to produce a higher quality beam than most other types of electron guns. These electron guns typically generate a large number of electrons by photoemission from a laser-illuminated photocathode located inside an accelerating structure. The accelerated electrons typically enter a resonant cavity having a resonant frequency f, exciting the electrons to higher energy. A high-current electron beam is thus generated at an output port of the resonant cavity for injection into a high-power accelerator.
The optical frequency ν of the laser illuminating the photocathode must be chosen so that the incident photon energy hν is larger than the work function of the photocathode material. The work function is a property of the emitting surface of the photocathode. The choice of laser, therefore, is dependent on the photocathode materials available. Unfortunately, the more reliable photocathode materials typically require more intense and higher frequency laser illumination. A reliable, efficient, long-life high power laser and photocathode combination capable of generating high-current low-emittance electron beams is not known in the prior art.
In addition, high radio frequency (RF) power is required to generate a high accelerating RF field at the photocathode in a high-energy particle accelerator. In those accelerators equipped with normal conducting RF cavities, therefore, the RF guns are limited to pulsed operation with a low duty cycle, typically below 10−4. There have been attempts to overcome this limitation by using a superconducting acceleration cavity, which in principle enables operation in a continuous wave (CW) mode with the same beam quality.
RF photoinjectors with superconducting cavities operating in CW mode, therefore, are desired for use in high-average-current injectors. The superconducting cavity can advantageously maximize the electric field for good emittance properties and minimize power consumption. The sensitivity of the superconducting cavity, however, imposes even more constraints on the photocathode. For example, in order to preserve the high field characteristics of the cavity, the photocathode must not contaminate the cavity with particles from the photoemissive layer. In addition, the photocathodes must be characterized by a high quantum efficiency (QE) and long life time. The heat load imparted to the photocathode by the laser and the high electric fields must also be efficiently transferred from the photocathode, to allow an electron bunch to be emitted from the cathode with low thermal emittance.
There is a need, therefore, which is lacking in the prior art, for a reliable and efficient long-life electron gun for the generation of high-current high-brightness electron beams. There is a particular need for long-life, non-contaminating cathodes, especially photocathodes, which can be used in superconducting RF electron guns for the generation of high-current high-brightness electron beams.